Combining last names for baby

My spouse and I have different last names, and we can't decide what to do for future children's last name. Hyphenating seems like it would be a major pain for them. I was thinking of maybe coming up with a new name using both our names. Eg. Martin + Bentley = Marley or Bentin. Is this weird?

It certainly isn't common, but it's not completely unheard of. I know several people who have combined last names of their respective parents. If you live in a place where it's much more common that parents and kids have the same last name, you may find yourself explaining the last name often, but that's hardly a reason not do combine the names if that's what you want. It's perfectly reasonable that you and your spouse would want your kids' last name to reflect your own individual names.

Plus, it gives you more name options for the kids - if you don't like how a first name sounds with one last name, try it with the other!

Lineage and family history are very important to me, so this isn't something I would consider (inventing a new last name). I also strongly dislike hyphenated names, so in your shoes I would give one of the last names as a middle name and the other as the true last name (in your example above, it would work perfectly). JohnAndrewMartinBentley or AnnaRoseBentleyMartin etc. Or you could get rid of the second name if it feels like a mouthful (so JohnMartinBentley or AnnaBentleyMartin). I also have a different last name than my spouse, but the children have his last name only. It was more important to him than it was to me.

A new name is really weird to me, but in my country that wouldn't even be possible. I strongly dislike the idea, to me a lastname is heiratage, it has a story somehow. Since your family names are both short and easy I think hyphenating would be fine. At least it is way better than the new name idea.